ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-10-3855-2010Impact of dust on tropospheric chemistry over polluted regions: a case study of the Beijing megacityZhuS.12ButlerT.2SanderR.2MaJ.1LawrenceM. G.231Chinese Academy of Meteorological Sciences, Beijing, China2Max Planck Institute for Chemistry, Mainz, Germany3University of Mainz, Mainz, Germany2604201010838553873This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/10/3855/2010/acp-10-3855-2010.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/3855/2010/acp-10-3855-2010.pdf

The box model MECCA (Module Efficiently Calculating the Chemistry of the
Atmosphere) is extended by incorporating detailed heterogeneous chemistry
occurring on mineral aerosol surfaces. The model is used to investigate the
impact of dust on tropospheric photochemistry, when the dust is transported
to a polluted region, focusing on the example of Beijing. The impacts of
dust via heterogeneous removal of gases are analyzed for different
hypothetical transport rates, which are described by four different exchange
rate coefficients <i>K<sub>t</sub></i> in the model. Along with the dust, airmasses with
trace gas levels characteristic for regions upwind of Beijing are
transported with the same rate (<i>K<sub>t</sub></i>). Substantial impacts are found for
many gases, including O<sub>x</sub> (O<sub>3</sub>+O(<sup>3</sup>P)), NO<sub>x</sub> (NO+NO<sub>2</sub>)
and OH. The O<sub>x</sub> daily average mixing ratio decreases due to
heterogeneous reactions on dust. The change ranges from &minus;2.5 to
&minus;18.4 nmol mol<sup>&minus;1</sup>, and is larger for faster mixing with upwind air masses
(i.e. greater <i>K<sub>t</sub></i>). This translates into a large relative change in O<sub>x</sub>,
ranging from &minus;44% to &minus;55%, depending on <i>K<sub>t</sub></i>. By assuming an
artificial 50% decrease of all photolysis rates, the impacts of dust via
perturbation of the photolysis rates in the polluted region are also
estimated. Furthermore, the uncertainties in the results due to the
uncertainties in the uptake coefficients are evaluated. It is found that for
all gases which are heterogeneously removed, the self-removal results in the
largest uncertainty (e.g. &minus;49% for O<sub>3</sub>, &minus;76% for NO<sub>2</sub>, &minus;47%
for HNO<sub>3</sub>, &minus;92% for HCHO, &minus;64% for CH<sub>3</sub>OH and &minus;93% for
SO<sub>2</sub>). The heterogeneous removal of NO<sub>2</sub> is found to be particularly
important, because it results in significant levels of uncertainty not only
for itself, but also for OH (340%) and HO<sub>2</sub> (365%). Moreover, the
heterogeneous removal rates of HCHO and O<sub>3</sub> also have farther-reaching
effects on the OH concentration (resulting in changes of &minus;55% and 45%,
respectively), and the heterogeneous removal of HCHO results in an
uncertainty of &minus;38% in the HO<sub>2</sub> concentration. The limitations of
MECCA due to its missing oxidation mechanism for aromatics and other higher
VOC species has also been considered, and shown to be potentially important
in the quantitative results, though not likely to change the qualitative
results of this study.